Alien bivalve molluscs occurrence, population dynamics and impact: Lake Maggiore (Italy) and Danube River (Bulgaria) case studies

Nowadays, the presence of alien species is a worldwide problem, especially when the presence of the new species has the character of a biological invasion which threatens biological diversity and affects ecosystem services thus causing environmental, economic and social damages. The freshwater bivalve Dreissena polymorpha is included among the 100 World's Worst Invasive Alien Species (IUCN/SSC ISSG), and Corbicula fluminea and D. polymorpha are listed among the 100 Worst Invasive Alien Species in Europe (DAISIE). Further, the Chinese Pond Mussel Anodonta woodiana and the Quagga Mussel Dreissena rostriformis bugensis are becoming globally spread in last decades, causing biodiversity and economic losses. Here we show how different environments, such as a big subalpine lake, Lake Maggiore (Northern Italy) and the second longest river in Europe, Danube River (Bulgaria), are affected by similar problems: the invasion by non-native freshwater bivalves; and how these different habitats influence the occurrence and population dynamics of the invaders. Presence-absence maps have been prepared to present the distribution of the bivalve alien species (C. fluminea, D. polymorpha, A. woodiana and D. rostriformis bugensis) in the two different freshwater bodies, Lake Maggiore and the Danube River (Bulgaria). In addition, to evaluate the potential impact of C. fluminea, which is rapidly becoming the dominant littoral species in Lake Maggiore, a field and mesocosm study of its population dynamics and reproductive cycle is presented. The results from the study of quantitative parameters and habitat characteristics of C. fluminea and other alien bivalves in the Bulgarian sector of the Danube River and its tributaries are also presented.